The present invention relates to air cells, and, more particularly, to an air cell having a porous sheet improved so that it shows a satisfactory discharging performance even after a long period of storage.
In batteries which are provided with a gas diffusion electrode and which use oxygen as an active material, there is provided a fuel cell and an air cell; those cells will be hereinafter called "air cells". In an air cell that uses an alkaline solution or neutral solution as electrolyte, particularly, vapor enters and leaves through the gas diffusion electrode (oxygen electrode) in accordance with a change in the internal vapor pressure. This vapor action changes the concentration and volume of the electrolysis solution in the cell, affecting various characteristics of the cell.
The above situation will now be explained referring to a button type air cell shown in FIG. 1. In FIG. 1, reference numeral "1" denotes a gas diffusion electrode which is an oxygen electrode (air electrode), and reference numeral "2" is a porous film which supports the gas diffusion electrode 1, both made of polytetrafluoroethylene that diffuses gas but shuts off a liquid. Reference numeral "3" denotes an air-diffusing porous sheet, reference numeral "4" an air-inlet hole, reference numerals "5" and "6" separators, reference numeral "7" a zinc negative electrode, reference numeral "8" a negative container, reference numeral "9" an insulated gasket, and reference numeral "10" a positive container.
A potassium hydroxide solution with a concentration of 30 to 35% is typically used as the alkali electrolysis solution. If the relative humidity is higher than 47 to 59%, therefore, the electrolysis solution absorbs outside moisture, lowering its concentration and expanding its volume. This reduces the discharging performance and causes the electrolysis solution to leak. If the relative humidity is less than 47%, on the other hand, the electrolysis solution is vaporized, increasing the internal resistance and lowering the discharging performance. As air cells are likely to be significantly affected by the environmental conditions, they do not have an excellent characteristic for long storage and are designed just for a certain field accordingly. The air cells must surmount this problem to achieve a variety of applications.
To overcome those shortcomings, various measures have been taken. For instance, a material that reacts with the electrolysis solution is inserted to the portion of the peripheral portion of the air hole to prevent the electrolysis solution from leaking outside the cell. Alternatively, an electrolysis-solution absorbing member, such as paper, or a nonwoven fabric made of a polymer material, is provided to prevent the electrolysis solution from leaking outside the cell. Further, the air hole is made very small to restrict the amount of the oxygen supply, thereby inhibiting vapor or carbon dioxide gas from permeating the cell. None of those methods can accomplish either a sufficient performance to prevent the leakage of the electrolysis solution or a sufficient discharging performance, particularly, a satisfactory long-discharging performance.
The main causes for such leakage or evaporation of the electrolysis solution are the dilution and volume expansion of the electrolysis solution due to the vapor in the air permeating the cell, and the inhibition of the discharge reaction and blocking of the air passage by the production of a carbonate from the carbon dioxide gas. When the outside humidity is low, flying off of water in the electrolysis solution reduces the aforementioned performances. Proposed solutions to eliminate these problems include a method of supplying air to the oxygen electrode via a film which suppresses the permeation of vapor or carbon dioxide gas to ensure selective permeation of oxygen by priority over it, e.g., via a non-porous uniform thin film of a polysiloxane type material, and a method of using a composite film consisting of a thin film of a polyorganosiloxane copolymer and an alkali resistant, minute-porous film, or a composite film consisting of a thin film of metal oxide or an organic compound containing metal atoms and a proper porous film.
However, those methods cannot yield selective oxygen permeability or sufficiently inhibit the permeation of vapor or carbon dioxide gas, so that air cells showing a satisfactory discharge characteristic have heretofore not been realized.